During hydrocarbon extraction operations, safety equipment is installed for utilisation in the event of catastrophic failure to prevent damage to human life and the environment. This is particularly the case for sub-sea hydrocarbon extraction where the presence of water can carry contamination from an oil well many thousands of miles potentially causing huge environmental damage.
The primary barrier utilised to shut a well is the blow out preventer which sits on the well head. For a subsea well, a riser links the oil rig to the blow out preventer, the riser allowing the passage of drilling and completion tools from the oil rig to the oil well through the blowout preventer. In the event of a catastrophe, it is beneficial to be able to sever drill pipe and the like within the riser to, first, permit successful detachment of the rig from the well head and, second, allow the severed tubular to drop below the closure mechanism of the blowout preventer, allowing the blow out preventer to close more easily.
The use of charges to sever tubulars has been previously described. These charges are generally in the form of a linear shaped charge which creates a blade of plasticised metal which is directed at the targets to be severed.
It has been found, however, that linear shaped charges, particularly when closing in on a tubular target, lose energy as they pass through the medium between the charge and the tubular element, and coalescence of adjacent charge material as the charge material converge on the target result in uneven impact on the target, with resulting non-uniform and inconsistent cutting. To overcome these problems, high amounts of explosives are required making the procedure more dangerous and costly than it otherwise would be.
Furthermore, where multiple shaped charges are used, there are detonation problems where the charges are in close proximity. Conventional detonation of multiple charges effectively happens sequentially and this can have an adverse effect on the cutting of the target to the extent that severance may not be achieved. The adverse effect may be caused by the jets of material coming together or impacting on the target at different times. Furthermore, non-simultaneous detonation by the trigger mechanism can result in the shockwave generated by one charge reaching and triggering an adjacent charge before the detonation signal has reached the adjacent charge.